Acids and Bases
What Are the Differences between Acids, Bases, and Buffers?
Teacher Note: Connections
In this concept, students will study the characteristics of solutions by using models to predict their behaviors. They analyze a system by defining its boundaries and initial conditions, as well as its inputs and outputs. Engage students using the “Twenty Questions” strategy by displaying images of models of acid and basic solutions and having students pose questions. The “Twenty Questions” strategy is found on the Professional Learning tab. Click on Strategies & Resources, then Spotlight On Strategies (SOS). "Twenty Questions" is found underneath “Questioning.”
As students read and comprehend complex texts, view the videos, and complete the interactives, labs, and other Hands-On Activities, have them summarize and obtain scientific and technical information. Students will use this evidence to support their initial ideas on how to answer the Explain Question or their own question they generated during Engage. Have students record their evidence using “My Notebook.”
Acids: Hydrogen Ion Donor
Teacher Note: Misconception
Students may think that substances that are more acidic have a higher pH level. In fact, a lower pH level indicates that a substance is more acidic.
An acidglossary term (opens in a new window) is defined as a molecule (compound) or ion that has the ability to either donate a proton or hydrogen ion H+, or form a covalent bond with an electron pair. Acids play an important role in the chemistry that affects a person’s daily life. Between the foods a person eats, the products used throughout the day, and the body’s internal chemistry, acids are everywhere.
There are a few definitions that have been developed to describe the chemistry of acids. In the first category, in the late 19th century, Swedish chemist Svante Arrhenius defined acids as compounds that dissociate into hydrogen ions (H+) and a negative ion when dissolved in water. Along similar lines of thought, in the early 20th century, chemists Johannes Brønsted and Thomas Lowry more broadly defined acids as hydrogen ion donors.
In the second category, the acids form a covalent bond with an electron pair. This was proposed by Gilbert Lewis in 1923, and therefore such acids are referred to as Lewis acids. An example of a Lewis acid would be ammonia (NH3), where nitrogen forms a covalent bond with hydrogen by sharing a lone pair of electrons. In nature however, Arrhenius or Brønsted–Lowry acids are more commonly found than Lewis acids.
Despite the different definitions that have been developed, acids have a distinctive set of properties.
- Sour or tart: Foods such as vinegar and lemons have a sour or tart flavor because of acids. Vinegar contains acetic acid. Lemons and other citrus fruits contain citric acid.
- Corrosive: Acids react with some compounds to produce gases while corroding the compound. Acids react with certain metals to produce hydrogen gas. They also react with carbonates to produce carbon dioxide gas. And they react with bases to form salts.
- React with indicators like litmus paper/solution: When acids are mixed with certain compounds called indicators, they change the colors of the indicators. The color of an indicator can be used to estimate the strength of the acid. For example, acids turn blue colored litmus solution into red color. The intensity of the color depends on the strength of the acid.
Teacher Note: Connections
The pH scale is a logarithmic scale. Help students understand the significance of how quantity impacts pH levels by graphing pH onto a linear scale. This also allows students to use algebraic thinking to examine scientific data and predict the effect of a change in one variable on another. Have them plot how many excess protons there are at pH 1 (1 in 10 water molecules), at pH 2 (1 in 100 water molecules), and so on. This can be a class activity because you will need a very large piece of paper to get to pH 14!
Bases: Hydrogen Ion Acceptor
A baseglossary term (opens in a new window) is defined as a molecule that has the ability to accept protons from a proton donor and produce hydroxideglossary term (opens in a new window) ions (OH-) in aqueous solutions. Common examples of bases are hydroxides of the alkali metals (NaOH) and alkaline earth metals (Ca(OH)2). Much like acids, bases are common in daily life. They are found in foods, medicines, cleaning products, and inside the human body. According to Arrhenius’s definition, a baseglossary term (opens in a new window) is a substance that dissociates into a hydroxide ion (OH-) and a positive ion in a solution. This definition proved troublesome for certain bases such as sodium carbonate and ammonia. Some substances are bases, but not hydroxides (also called alkalis). The Brønsted–Lowry definition of bases as hydrogen cation acceptors (H+) is, therefore, more inclusive.
Bases have the following distinct properties.
- Bitter: Foods such as tonic water have a bitter taste because of bases. Tonic water contains the base quinine.
- Slippery to the touch: Many soaps and detergents are bases. The slippery feel of these compounds is a result of bases they contain.
- React with indicators: Like acids, when bases mix with indicators like litmus, they change the color of the indicators (red litmus into blue, which is opposite to what an acid does). The color of an indicator can be used to estimate the strength of the base.
- Acids and bases react with each other in a neutralization reaction typically producing salt and water.
Teacher Note: Misconception
Students may think that concentration and strength are synonymous, but they represent different properties of acids and bases. Concentration refers to the amount of solute present in a solution, while strength refers to the amount of ions that dissociate in a solution. For example, hydrochloric acid (HCl) is a strong acid because nearly all of its molecules ionize into H+ and Cl-. This property does not change whether it is concentrated or diluted in solution.
The pH Scale
The pHglossary term (opens in a new window) is a numeric scale used to measure the acidity or basicity of a given compound. Acids and bases have varying degrees of strength. For example, humans can safely ingest moderate amounts of citric acid because it is a weak acid. However, ingesting any amount of a strong acid like hydrochloric acid would be extremely dangerous. Ingesting strong acids would burn oral tissue and cause severe damage, perhaps even death. The strength or weakness of an acid depends on how completely it ionizes in water. This is measured by the amount of H+ or OH– produced.
The strength of acids and bases is related to the scale used to measure them. The measure of a substance’s acidity or basicity (also called alkalinity) is its pHglossary term (opens in a new window), which is an abbreviation for “potential of hydrogen.” Under normal conditions, the pH scaleglossary term (opens in a new window) ranges from 0 to 14. The values on the pH scale represent the negative logarithm of the H3O+, or hydronium ionglossary term (opens in a new window), which depends on the concentration of the substance.
A substance with a pH of 7 is neutral and is neither acidic nor basic. The pH of pure water is 7. A substance with a pH less than 7 is an acid. Acids measuring close to 0 are very strong acids, while weak acids have pH values near 7. A substance with a pH greater than 7 is a base. The strongest bases have a pH near 14, while weaker bases measure close to 7.
| Numeric Value | pH range |
| Highest Acidic | < 3.5 |
| Extremely Acidic | 3.6–5.0 |
| Moderately Acidic | 5.1–6.5 |
| Weakly Acidic | 6.6–7.0 |
| Neutral | 7.0 |
| Weakly Basic | 7.1–7.9 |
| Moderately Basic | 8.0–8.5 |
| Extremely Basic | 8.6–8.9 |
| Highest Basic | 9.0–14.0 |
Reactions with pH indicators allow scientists to use color to approximate pH values. Many pH scales are color coded according to changes in indicator colors. To use these scales, scientists mix an acid or base with an indicator and observe the color change. Then, they compare the color of the mixture to the range of colors on a printed pH scale and estimate the pH of the solution. This method is not exact, but it gives a good approximation of pH values. A common indicator for this practice is pH or litmus paper, which is a small strip of paper containing certain indicator compounds.
Neutralization and Buffers
A common question asked by many students is: What happens if an acid and a base are mixed together? Mixing an acid with a base results in a neutralization reaction. When acids and bases of equal strength are combined, their ions combine to form an ionic compound, or salt and water. The resulting solution is neutral. For example, when a strong acid like hydrochloric acid (HCl) reacts with a strong base like sodium hydroxide (NaOH), the H+ cation of the acid combines with the OH- anion of the base to form water (H2O) while the cation of the base (Na+) and anion of the acid (Cl-) forms sodium chloride (salt). The reaction can be shown as:
HCl + NaOH = H2O + NaCl
However, not all acid–base mixtures result in a neutral solution. For example, a strong acid mixed with a weak base would result in a weaker acid, but not complete neutralization, as exemplified by the reaction of hydrochloric acid (HCl) with ammonia (NH3): HCl + NH3 = NH4+ + Cl-, where NH4+ is the conjugate acid of the weak base NH3.
Teacher Note
Discuss with students how the formula can be used to determine the concentration of the solution in the beaker. Ask students what purpose the indicator serves in the apparatus setup.
Combining acids and bases is commonly done to produce buffers. A bufferglossary term (opens in a new window) is an aqueous solution of a weak acid and its conjugate base, or vice versa. Buffers are able to resist drastic changes in pH when small amounts of acids or bases are added to them. For example, suppose hydrochloric acid (HCl) is added to a buffer made up of ammonia (NH3), a weak base, and its acid, ammonium (NH4+). The ammonia takes up the H+ released from the strong hydrochloric acid and becomes NH4+. Since the weak base immediately takes up the hydrogen ions, they do not significantly contribute to the pH of the buffer solution, and the pH remains mostly unchanged. This buffer would work the same way if a strong base like sodium hydroxide were added (NaOH). Instead, the ammonium would donate H+ to the base, forming ammonia and water. Buffers are able to achieve resistance to pH changes because they form an equilibrium condition between an acid and its conjugate base or a base and its conjugate acid.
Buffers are important biological solutions. Most organisms must keep a constant internal pH level in order to maintain homeostasis and stay healthy. For this reason, buffers in the blood act to neutralize acids and bases that threaten to change the body’s pH level. For example, the bicarbonate buffering system is used to regulate the pH of blood.
Teacher Note: Practices
In this collection, students will consider the behavior of acids and bases, and analyze data given regarding biological buffers to determine the relevance of these solutions to living organisms. Consider using a strategy like the Jigsaw strategy before students complete these items in order to help students assemble the new information they have learned regarding acids and bases. This strategy is found on the Professional Learning tab. Click on Strategies & Resources, then Spotlight On Strategies (SOS). “Jigsaw” is found underneath “Inference and Prediction.”
